1. RAPID AND SLOW COMMUNICATION OF OVERTURNING
CHANGES OVER THE NORTH ATLANTIC
V. Roussenov and R. Williams
Oceanography Laboratories, University of Liverpool, U.K.
1. Motivation
We study the different mechanisms by which overturning changes are communicated over the basin
rapid response: wave propagation Kelvin waves, coastal trapped waves (Huthnance, 1978; Johnson & Marshall, 2002)
slower response: advection along western boundary, local recirculation and evolution of dense water masses
The study links to the UK NERC RAPID monitoring programme
2.Model experiments
Isopycnic model ( MICOM ) is integrated over realistic topography and driven by annual mean wind stress and buoyancy forcing (relaxation to Levitus data at high latitudes). 100 years spin-up
Twin experiment (10 years): standard vs. increased high latitude buoyancy forcing
Advection is monitored by a transient model tracer, released in Labrador Sea when the perturbed buoyancy forcing starts.
coarse resolution
1.4o – 0.7o
fine resolution
0.28o – 0.14o
Depth [m] Sea-surface height [cm]
Perturbed experiment
Increased buoyancy forcing simulated by shallowing of deep interfaces by 120m/10 days
layer interface depth along 35oN
3. Slow advective response
Dense fluid is formed in the Labrador basin and spreads equatorwards along a deep western boundary current, as revealed by releasing an idealised transient tracer for 10 years.
4. Rapid response
In the perturbed case, the increased buoyancy loss in the high latitudes leads to
initially a pressure signal rapidly propagating along western boundary on a timescale of several weeks to months.
an intermediate response involving changes in local circulation and layer thickness, excited by the waves (timescale of several months to years).
eventually, the arrival of a transient tracer signal, after the increase in layer thickness and changes in sea-surface height.
year 0.5
year 1.5
year 2.5
Anomalies (perturbed-reference) in layer thickness and Labrador Sea
sea-surface height tracer
Layer 27.7 thickness [m] and transport (contours in Sv) Labrador transient tracer
Time series of layer thickness (solid lines) for the perturbed and reference case
Transient tracer (dashed lines)
W-E Hovmuller diagrams of deep layer interface depth anomalies (perturbed-reference)
N-S Hovmuller diagram of deep layer interface depth anomalies (perturbed-reference), N-S section following the western slope Labrador Sea transient tracer - contours
4.Conclusions
Overturning changes via 3 phases
rapid topographic waves
local advective changes
slower far-field advection
The communication of overturning signals will be examined within the UK NERC RAPID monitoring programme by deploying bottom pressure recorders and vertical profilers along the western boundary of the N. Atlantic [involving C. Hughes, P. Foden (POL), D. Marshall (Reading) and R. Williams (Liverpool)]
References
Huthnance J.M.,1978: On coastal trapped waves: analysis and numerical calculations by inverse iteration. JPO,8, 74-92
Johnson H.M. and D.P. Marshall, 2002:A theory for the surface Atlantic response to thermohaline variability. JPO, 32,
RAPID climate change thematic programme
Contact: